Resistive loading technique for antennas

ABSTRACT

HF antennas are matched to multicouplers efficiently by means of discrete, resistive loading. Antennas are loaded by electrically connecting selectively predetermined resistances at various places on the antenna to alter currents thereon and thus vary the input impedance of the antenna. Resistive loading at the apexes far from the feed point and connecting antenna apexes to ground through resistive loading provides maximum energy radiation and reduces energy reflection and undue energy losses.

United States Patent Goodbody Apr. 9, 1974 RESISTIVE LOADING TECHNIQUE FOR ,875,050 8/l932 Lindenblad 343/846 ANTENNAS FOREIGN PATENTS OR APPLICATIONS Inventor: Richard Goodbedy, San Diego, 736,836 9/1955 Great Britain 343/749 Calif. 563,493 8/1944 Great Britain 343/739 [73] Assignee: The United States of America as a represented by the Secretary of the f Lleberma! Navy, Washington DC. Attorney, Agent, or Firm-R. S. Sc1asc1a; G. .l. Rubens;

J. W. McLaren [22] Filed: Oct. 13, 1972 [21] Appl. No.: 297,541 [57] ABSTRACT HF antennas are matched to multicouplers efficiently 52 US. Cl 343/709, 343/739, 343/749, by means of discrete, resistive loeding- Antennas are 3 /88 5 loaded by electrically connecting selectively predeter- [5l] Int. Cl. HOlq 1/34 mined resistances at Various Places on the antenna to [58] Field of Search 343/739, 749, 802, 890, alter currents thereon and thus vary the input p 343 /891 dance of the antenna. Resistive loading at the apexes far from the feed point and connecting antenna apexes 5 References Cited to groundthrough resistive loading provides maximum UNITED STATES PATENTS energy radiation and reduces energy reflection and undue energy losses. 3,596,272 7/1971 Blonder 343/740 Bruce 343/749 3 Claims, 9 Drawing Figures PATENTEB APR 9 \374 SHEET 1 "BF 3 PATENIEBAPR 9 :914 I 380361 5 sum 2 or 3 iii MTENTEH m s W4 $803615 saw s-ur 3 30 2O lOdb *1 PF ON FORWARD FANS 20 K9 AT EACH APEX,

FANS

1 pF ON FORWARD 1O Kfl AT EACH APEX FIG. 8

RESISTIVE LOADING TECHNIQUE FOR ANTENNAS BACKGROUND OF THE INVENTION 7 Designers of naval communication systems for shipboard use must cope with the problem of installing and operating their systems in relatively crowded environments due to the limited topside space available on most ships. The problem is especially critical with HF antenna systems since they require considerable space for adequate operation primarily because of the long wavelengths involved relative to the dimensions of existing naval ships. For example, a DLG-type ship is only two wavelengths long or less at 4MHz; yet, HF antennas on this ship must compete for topside space with other important systems such as weapon launchers. Consequently, as the topside area on naval ships becomes more crowded, antenna system performance is inevitably degraded by the presence of nearby structures which can act as parasitic radiating elements.

In addition to the problem of physical crowding, meeting system operational requirements can also present problems. For example, HF broadband transmitting antennas aboard modern navy ships are typically required to have omnidirectional coverage and a 3:1 frequency band. These requirements and the complexity of a ships environment make it difficult to match the antenna input impedance to the multicoupler output impedance over the entire frequency band of interest. As is well-known, a mismatch between the antenna and multicoupler can cause the antenna to reflect a portion of the power into the multicoupler, thereby creating standing waves and reducing the efficiency of the communications system. Furthermore serious nulls can occur in the radiation patterns because of parasitic excitation of ship structures.

Presently various types of lossless, lumped L-C matching networks are used to match the antenna to the multicoupler within desired limits; However, in some cases it is impossible or infeasible to use a matching network alone to match the antenna to the multicoupler.

SUMMARY OF THE INVENTION A novel technique is disclosed for matching HF shipboard antennas to multicouplers in communications systems by means of discrete, resistive loading. Antennas are loaded at various selectively predetermined positions on the antenna structure to alter the currents on the antenna and thereby change the input impedance. The loading increases the antenna, improves the VSWR and the radiation patterns without causing excessive RF energy losses. Resistive loads are electrically connected at the apexes of twin-fan antennas far from the feedpoint so that the maximum energy is radiated before energy is dissipated as I R losses in the load. The apexes of antennas are further connected to ground through a resistive load to reduce energy reflections from the otherwise opened end without causing undue energy loss. Resistive loading can also be connected in the middle of an antenna or an antenna can be continuously loaded to improve other properties such as increasing the bandwidth. The resistive loading technique can be used advantageously as long as the distortion of the currents does not create unacceptable nulls in the radiation patterns or reduce the gain of the antenna below a minimum required level.

OBJECTS OF THE INVENTION It is the primary object of the present invention to provide a relatively simple and inexpensive technique for effectively matching communication antennas to associated multicoupler apparatus.

It is another object of the present invention to provide a technique that can be used to resistively load conducting and radiating elements located in the near field of antennas to improve the operational characteristics of the antenna.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 8 illustrates a radiation pattern of the twin-fan antennas of FIG. 5 at a frequency of 6-MHz; and,

FIG. 9 is an illustration of a parasitic structure connected to the mast of an antenna to improve the impedance and radiation patterns thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT The inventive concept to be disclosed herein essentially embodies the technique of using resistive loading for matching communication antennas to multicouplers on HF shipboard communication systems. The technique can be used advantageously to improve radiation patterns and impedance plots, and furthermore the technique can be extended by means of resistive loaded wires to damp currents on parasitically excited ships structures thereby resulting in marked improvement of both VSWR and radiation patterns with negligible power loss.

The concept will be described with reference to three methods of loading twin-fan antennas and parasitic structures as shown in FIGS. 1, 5 and 9. In the first method, antennas are loaded at the apexes far from the feedpoint so that the maximum energy is radiated before energy is dissipated in the form of FR losses by the loading resistors. In this method, a resistor is connected between the apex of the antenna and ground so that energy reflections from the otherwise open end are reduced without causing undue energy loss.

In the second method, resistive loading is applied at the apex as above and at randomly selected points on the conducting wires of the fan antenna to improve other properties of, the antenna such as increasing the bandwidth thereof. In the third method, apex and random resistive loading as above are used, and furthermore, parasitic structures in proximity to the antenna operationalv environment are resistively loaded to ground. 1

In general, it can be stated that if the distortion of the currents created by resistive loading as disclosed herein does not generate unacceptable nulls in the radiation pattern or reduce the gain of the antenna below a minimum required level, then the technique can be used to advantage. In all three approaches it can be seen that discrete resistive elements are attached to various places on the antenna or on nearby conducting and radiating structures to alter the currents on the antenna and thus change the input impedance.

FIG. 1 illustrates a typical twin-fan 2-6 MHZ transmitting antenna located and supported on the hull and superstructure 12 of a naval ship. All structure shown in FIG. 1 is conductive except that which is shown as being insulator. The superstructure 12 is connected through the ships hull to ground as is wellknown.

The antenna 10 comprises two, three-wire fans 14 which are connected through the insulators 16 to the yardarm 18. The yardarm 18 is in turn rigidly mounted to a vertical mast member 20. The antenna feedpoint 21 is situated directly below the cross created by the yardarm and the mast.

The opposite ends of the twin fans 14 are connected through the resistors 22 and 24 to the vertical support structures 26. The resistors are connected thru the conductive structure 26 to ground as above.

In effect, each of three wires of the fans is resistively loaded by the resistors 22 and 24 at the apexes farthest from the feedpoint 21. This type of loading asures that maximum energy is radiated before energy is dissipated as l R'losses by the resistors 22 and 24.

The impedance plots and the radiation patterns of FIGS. 2, 3, and 4 varied directly as the value of the loading was varied. Without any resistive loading whatsoever, FIG. 2 shows that the antenna 10 had an unacceptable maximum VSWR of approximately 7:1. With a IOO-ohm load comprising the resistors 22 and 24, the VSWR was not improved but the impedances were altered, thereby indicating that in some cases the 100- ohm load might change an unmatchable antenna into a matchable one. i With a 470-ohm load (not shown) the VSWR was greatly improved to approximately 3.7:l which would be relatively easy to match with a lossless matching network. With a l-kilohm load, the VSWR was measred at 3.8:1, and with a lO-kilohm load, the VSWR was 4.4:l. With a -kilohm load as shown in FIG. 3, VSWR deteriorated to 7.5:]

The radiation patterns shown in FIG. 4 for 6-MHz are plotted on a logarithmic scale, and the maximum is lO-db above the maximum power from a quarter wavelength monopole above an incident ground plane. The radiation patterns for no-load, load, and IOO-ohm and 470-ohm load configurations, the antenna radiates less energy or has less gain than in the no-load case. This confirms that considerable energy is dissipated in the load resistors.

With a l-kilohm load, the antenna gain is improved, but it is still too low for an effective antenna system. With loads of 10- and 20-kilohms, the gain of the antenna is only slightly less than in the no-load case for 2-MI-Iz and 3-I-IMz, but it is as much as 5-db less above S-MI-Iz. Obviously a trade-off decision would be required to determine whether the resulting power loss is justified to achieve an antenna that can be matched to within a 3:1 VSWR.

The structure shown in FIG. 5 represents a 2-6 MI-Iz transmitting antenna comprising a yoked, pair of twinfan antennas 32 and 33 which are generally suppored by the masts 36 and 44. The normal apex loading of the antennas is capacitive.

The twin-fan antennas 32 and 33 comprise three-wire antennas 34 and 35, respectively, as shown in FIG. 1 and FIG. 5. The wires 34 and 35 are connected at the top thereof to a yardarm structure through the insulators 38 and 39, respectively. The antenna wires 34 and 35 are connected to the feedpoints 40 and 46, respectively, and the wires 34 and 35 are resistively loaded at the far apexes by the loads 42 and 48, respectively. The resistive loads 42 and 48 are connected to ground as the resistors 22 and 24 above.

FIGS. 6 and 7 represent imepdance plots for the antennas of FIG. 5 with various resistive loads connected as above, and FIG. 8 is a radiation pattern for a frequency of 2-MHz.

Again it is clear that for lower values of resistors, excessive amount of energy is lost as IR losses in the resistive loads although the VSWR is improved. With a 10-kilohm load at each apex, the radiation patterns are more nearly omnidirectional; that is, the patterns have more shallow nulls at two frequencies than in a capacitive-loaded antenna and resistive loss appeared to be less than l-db over the frequency range of interest. For a ZO-kilohm load, the distortion of the radiation patterns was not significantly less than in the capacitiveloaded case.

FIG. 9 illustrates a twin-fan antenna 54 having fans 56 and 60 generally mechanically and electrically connected as in FIG. 5 and wherein, the fan wires 56 and 60 are resistively loaded at the apex as before by the resistors 76 and 78, respectively. That is, FIG. 9 illustrates a complex antenna environment wherein a parasitic structure is loaded (in addition to the apex loading if desired) to improve antenna impedance and radiation patterns.

In FIG. 9, the parasitic structure comprises a radar unit 74 and its support structures and which is located in the operationl environment or near-field of the antenna 54. As can be seen the radar unit 74 is loaded by a resistive load 72 connected between it and the mast 62 of the antenna. The mast 62 is connected to ground as previously described.

Although the wires of the antennas 56 and 60 are shown with resistive loads as in FIGS. 1 and 5, it should be understood that they could be loaded at experimentally determined random locations on the wires at the option of the antenna designer faced with an operational environment which required it.

Essentially, an experimental technique was used on the system of FIG. 9, by varying both the surroundings in the near field, such as by adding resistive loaded wires between the mast and the platform and by apex loading to improve the input impedance and the radiation pattern of the antenna. The added loaded wire changes the forced currents that normally flow on a ship.

Experiments verified that the VSWR may be improved by resistive loading and/or by changing the surroundings such as by adding resistive loaded wires between mast and platforms. The mutual coupling between these currents on the structure and those on the wire fans dictate that the, radiation pattern and input impedance will change as a function of frequency and the loading. Because of the complexity of these structural situations the technique must be an experimental one for each particular configuration.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings, It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. ln shipboard broadband communication systems, a method for matching system fan antennas to system multicouplers and comprising a plurality of conductive wires connected at one end to a common apex and each of said wires radiating therefrom ina fan-like manner to an energy feed connection comprising the steps of electrically connecting discrete, resistive elements atvarious selectively predetermined places on the conductive wires of said antennas to alter the currents thereon and thereby vary the input impedance of said antennas, said various places being randomly selected to provide a desired input impedance.

2. A method for improving the VSWR levels a'rididiation patterns of a fan antenna having aplurality of conductive wires connected at a common apex and connected at the other end to an energy feed connection in a spaced manner with respect to each other comprising the steps of electrically connecting selectively predetermined, discrete first resistive "loads be tween said antenna apex farthest from the energy feedpoint and ground to provide maximum energy radiation and to reduce energy reflections, and electrically connecting selectively predetermined, discrete second resistive loads at randomly selected places on the conductive wires of said antenna to alter currents therein and thereby vary the input impedance thereof.

3. A method for improving the radiation pattern of a shipboard transmitting fan antenna wherein parasitic structures are located in the antennas near-field and comprising the steps of connecting resistive loading at randomly selected positions on the conductive wires of said antenna, connecting resisitve loads between the apex of said wires, farthest from the energy feedpoint of said antenna, and ground, and further electrically connecting resistively-loaded conducting wires between randomly selected places on said parasitic structure and ground. 

1. In shipboard broadband communication systems, a method for matching system fan antennas to system multicouplers and comprising a plurality of conductive wires connected at one end to a common apex and each of said wires radiating therefrom in a fan-like manner to an energy feed connection comprising the steps of electrically connecting discrete, resistive elements at various selectively predetermined places on the conductive wires of said antennas to alter the currents thereon and thereby vary the input impedance of said antennas, said various places being randomly selected to provide a desired input impedance.
 2. A method for improving the VSWR levels and radiation patterns of a fan antenna having a plurality of conductive wires connected at a common apex and connected at the other end to an energy feed connection in a spaced manner with respect to each other comprising the steps of electrically connecting selectively predetermined, discrete first resistive loads between said antenna apex farthest from the energy feedpoint and ground to proviDe maximum energy radiation and to reduce energy reflections, and electrically connecting selectively predetermined, discrete second resistive loads at randomly selected places on the conductive wires of said antenna to alter currents therein and thereby vary the input impedance thereof.
 3. A method for improving the radiation pattern of a shipboard transmitting fan antenna wherein parasitic structures are located in the antenna''s near-field and comprising the steps of connecting resistive loading at randomly selected positions on the conductive wires of said antenna, connecting resisitve loads between the apex of said wires, farthest from the energy feedpoint of said antenna, and ground, and further electrically connecting resistively-loaded conducting wires between randomly selected places on said parasitic structure and ground. 